1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device increasing an aperture ratio thereof.
2. Discussion of Related Art
A liquid crystal display (LCD) device has first and second substrates and liquid crystals interposed between the first and second substrates. The LCD device also has first and second electrodes in the first and second substrates, respectively, to apply an electric field to the liquid crystals. The LCD device uses optical anisotropy and the polarization properties of liquid crystal molecules to produce images. Liquid crystal molecules have a definite alignment direction as a result of their long, thin shapes. This alignment direction can be controlled by the applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to optical the anisotropy, the refraction of incident light depends on the alignment orientation of the liquid crystal molecules. Thus, by properly controlling the applied electric field by the first and second electrodes, a desired image can be produced.
While various types of liquid crystal display devices are known, active matrix LCDs having thin film transistors and pixel electrodes arranged in a matrix are probably the most common. This is because these active matrix LCDs can produce high quality images at a reasonable cost.
As widely known, the LCD device has pixel electrodes in a lower substrate and a common electrode in an upper substrate. When electric signals are applied to the pixel electrodes and the common electrode, the electric fields generated are perpendicular to the lower and upper substrates. These electric fields control the arrangement of the liquid crystals to display images. However, the LCD device does not emit any light such that an additional light source is needed. Therefore, the LCD device includes a rear backlight device as its light source. The backlight device consumes approximately 60% of electric power supplied to the LCD device. In order to increase the luminance of the LCD device, the brightness of the backlight is commonly increased. However, increasing the brightness of the backlight increases the power consumption. Thus, the aperture ratio of the LCD device must be improved to increase the luminance of LCD device without increasing the power consumption.
To improve the aperture ratio in the LCD device, the size of the black matrix in the LCD device should be reduced. The black matrix size is generally defined by considering an alignment margin when attaching the lower and upper substrates and the prevention of light leakage caused by the liquid crystal molecules over data and gate lines. Therefore, the black matrix is positioned to cover the peripheral portions of the pixel electrodes.
Today, LCD devices employ benzocyclobutene (BCB) as an insulator because BCB has a low dielectric constant and improves the aperture ratio. A related art LCD device having BCB as an insulator will be explained with reference to FIGS. 1 to 4.
FIG. 1 is a partial plan view illustrating an array substrate of a related art LCD device.
FIG. 2 is a partial plan view illustrating a color filter substrate of the related art LCD device.
Referring to the array substrate of the related art LCD device in FIG. 1, gate lines 11 are formed in a horizontal direction and a gate electrode 12 extends from each gate line 11. Data lines 14 are then formed in a longitudinal direction. The data lines 14 cross the gate lines 11 so as to define a pixel region P. A source electrode 15 extends from each data line 14, and a drain electrode 16 is positioned opposite to the source electrode 15 across the gate electrode 12. The source and drain electrodes 15 and 16 overlap opposite ends of the gate electrode 12. A semiconductor layer 13 including an active layer of intrinsic semiconductor and an ohmic contact layer of extrinsic semiconductor is positioned over the gate electrode 12 and between the source electrode 15 and the drain electrode 16. Therefore, a thin film transistor T includes the gate electrode 12, the semiconductor layer 13 and the source and drain electrodes 15 and 16.
Still referring to FIG. 1, a pixel electrode 18 including a transparent conductive material is formed in the pixel region P. A portion of the pixel electrode 18 overlaps a portion of the drain electrode 16 and a contact hole 17 is formed in an overlapping area of the drain and pixel electrodes 16 and 18. The pixel electrode 18 contacts the drain electrode 16 through the contact hole 17. Both left and right peripheral side portions of the pixel electrode 18 overlap the data lines 14, respectively.
Referring to the color filter substrate of the related art LCD device in FIG. 2, a black matrix 21 has an opening 21 a that corresponds to the pixel electrode 18 of the array substrate. Each of the red, green and blue color filters 22a, 22b and 22c is formed on the black matrix 21 and in the opening 21a. Each color filter 22a, 22b and 22c corresponds to the pixel region P of the array substrate. Although not shown in FIG. 2, the color filter substrate includes a common electrode over all the color filters 22a, 22b and 22c. The common electrode is formed of a transparent conductive material.
FIG. 3 is a cross-sectional view taken along line III—III of FIGS. 1 and 2, and illustrates both the array substrate and the color filter substrate according to the related art.
Referring to FIG. 3, a gate insulation layer 32 is formed on a first substrate 31, and data lines 33 spaced apart from each other are formed on the gate insulation layer 32. A passivation layer 34 is formed on the gate insulation layer 32 while covering the data lines 33. The passivation layer 34 is made of an organic material, for example, benzocyclobutene (BCB), which has a low dielectric constant. On the passivation layer 34, pixel electrodes 35 that are made of a transparent conductive material are formed. Both the right and left side portions of each pixel electrode 35 overlap portions of the data lines 33. Although the pixel electrode 35 partially overlaps the data lines 33, signal interference between the pixel electrode 35 and the data line 33 does not occur because the passivation layer 34 has a low dielectric constant.
Still referring to FIG. 3, a second substrate 41 is positioned over the first substrate 31 and faces the pixel electrodes 35. On the rear surface of the second substrate 41, a black matrix 42 is formed. As described with reference to FIG. 2, the black matrix 42 has openings therein each corresponding to each pixel electrode 35. The black matrix 42 covers the peripheries of the pixel electrode 35 in order to prevent light leakage in regions except the pixel electrode regions. The width of the black matrix 42 covering the peripheries of the pixel electrode can be approximately the same as the width “a” that the pixel electrode 35 overlaps the data line 33. Red, green and blue color filters 43a, 43b and 43c are also formed, alternately, on the rear surface of the second substrate 41. The color filters 43a, 43b and 43c cover portions of the black matrix 42, and each of the color filters 43a, 43b and 43c corresponds to each pixel electrode 35. An overcoat layer 44 covers the color filters 43a, 43b and 43c to protect the color filters 43a, 43b and 43c from possible damages. The overcoat layer 44 also acts to planarize the rear surface of the color filter substrate. A common electrode 45 of a transparent conductive material is formed on the rear surface of the overcoat layer 44. A liquid crystal layer 50 is interposed between the first substrate 31 and the second substrate 41.
In the above-mentioned LCD device, since benzocyclobutene (BCB) has a low dielectric constant and is used for the passivation layer, signal interference is prevented between the pixel electrode and the data line although some of the pixel electrode and the drain electrode overlap each other.
However, the above-mentioned LCD device has a viewing angle problem. Namely, the LCD device having above-mentioned configuration and structure does not have a good viewing angle. Thus, various methods to overcome this disadvantage have been researched and presented. For example, a multi-domain LCD device is presented.
FIG. 4 is a cross-sectional view of an LCD device that adopts a multi-domain technology according to another related art.
Referring to FIG. 4, a gate insulation layer 62 is formed on a first substrate 61, and data lines 63 are formed on the gate insulation layer 62. A passivation layer 64 is formed on the gate insulation layer 62 to cover the data lines 63. For example, the passivation layer 64 is benzocyclobutene (BCB). On the passivation layer 64, pixel electrodes 65 are formed that are for example, a transparent conductive material. Auxiliary electrodes 66a and 66b are also formed on the passivation layer 64. The auxiliary electrodes 66a and 66b are spaced apart from each other and also spaced apart from the pixel electrodes 65. Although not illustrated in FIG. 4, the auxiliary electrodes 66a and 66b actually surround each pixel electrode 65 when viewed in superficial observation. The auxiliary electrodes 66a and 66b overlap the data lines 63 by a width “b”, respectively, but there is no signal interference between the data line 63 and the auxiliary electrodes 66a and 66b. 
Still referring to FIG. 4, a second substrate 71 is right above the first substrate 61. On the rear surface of the second substrate 71, a black matrix 72 is formed. The black matrix 72 has openings each corresponding to the pixel electrode 65. Thus, the black matrix 72 covers the auxiliary electrodes 66a and 66b and the peripheries of the pixel electrode. Color filters 73a, 73b and 73c that are red, green and blue are then formed on the rear surface of the second substrate 71, alternately. The red, green and blue color filters 73a, 73b and 73c fill in the openings of the black matrix 72 and cover portions of the black matrix 72. An overcoat layer 74 covers the color filters 73a, 73b and 73c to protect the color filters and to planarize the surface of the color filter substrate. A common electrode 75 is formed on the rear surface of the color filter layers 73a, 73b and 73c. The common electrode 75 is a transparent conductive material. Additionally, the common electrode 75 has a slit pattern 75a in a position corresponding to the pixel electrode 65. A liquid crystal layer 80 is interposed into an interval between the first substrate 61 and the second substrate 71.
The LCD device described in FIG. 4 has a slit pattern 75a in the common electrode 75 unlike the LCD device shown in FIG. 3. When voltage is applied to the pixel electrode 65 and the common electrode 75, a fringe field is produced between the pixel electrode 65 and the common electrode 75 because the common electrode 75 has the slit pattern 75a. The liquid crystals are arranged in two different directions with respect to the slit pattern 75a. Because of the different alignment directions of the liquid crystals, multi domains in which the liquid crystal molecules have different alignments can be obtained although a rubbing process is not conducted more than twice.
In the fringe-field multi-domain LCD device shown in FIG. 4, the auxiliary electrodes 66a and 66b act to strengthen the fringe field. Since the auxiliary electrodes 66a and 66b arc formed in the same plane as the pixel electrodes 65, the distance “c” between each of the auxiliary electrodes 66a and 66b and the pixel electrode 65 should be greater than or equal to 4 micrometers (μm) in order to prevent a short between the pixel electrode 65 and each of the auxiliary electrodes 66a and 66b. Additionally, in order to prevent the short between the auxiliary electrodes 66a and 66b, there should be a distance “d” between the two auxiliary electrodes 66a and 66b which is greater than or equal to 4 micrometers (μm). The width “e” of the auxiliary electrodes 66a and 66b is approximately 5 micrometers (μm). Therefore, in the fringe-field multi-domain LCD device shown in FIG. 4, the pixel electrode 65 is smaller than the pixel electrode 35 shown in FIG. 3, thereby decreasing the aperture ratio. Moreover, the black matrix covers the peripheries of the pixel electrode 65 by a width “f”. Considering the alignment margin when attaching the color filter substrate to the array substrate, the overlapping width “f” is defined to be approximately 5 micrometers (μm). Thus, the aperture ratio is further decreased in the LCD device shown in FIG. 4. Namely, the fringe-field multi-domain LCD device of FIG. 4 improves the viewing angle, but decreases the aperture ratio.